Three workers were killed in an accident at the weekend while unloading raw materials from a ship at a Nippon Mining & Metals-operated copper smelter in southern Japan.

The miners died on Saturday from what is believed to be a shortage of oxygen as they were unloading copper concentrate bound for the company’s Saganoseki smelter at a nearby port, a Nippon Mining official told Reuters.

Nippon has stopped unloading the copper concentrate while authorities investigate the cause of the accident.

“We have one week’s-worth of copper concentrate in stock at the smelter,” said the spokesman for Japan’s biggest copper producer.

The spokesman said the copper concentrate was supplied by Papua New Guinea’s OK Tedi copper mine.

The smelter was operating normally on Monday morning.